The present invention pertains to an inorganic light emitting diode light sheet and methods for manufacturing the same. More particularly, the present invention pertains to an inorganic light emitting diode light sheet that can be used as a photo-radiation source for applications including, but not limited to, general illumination, architectural lighting, novelty lighting, display backlighting, heads-up displays, commercial and roadway signage, monochromatic and full-color static and video displays, radiation-source for photo-curable materials, patterned light emissive images, and the like. Further, the present invention pertains more particularly to an inorganic light active sheet that can be used as a light-to-energy device for converting photo-radiation to electrical energy for applications including, but not limited to, solar panels, CCD-type cameras, photo-sensors, and the like. Further, the present invention pertains more particularly, to methods for mass-producing the inventive light active sheet at relatively low cost.
Inorganic light emitting diodes (LED) are based on elements of the periodic table of a vast variety. They come out of semiconductor technology, and indeed, a semiconductor diode such as a silicon diode, or a germanium diode were among the first semiconductor devices. These were made by doping the silicon or the germanium with a small amount of impurity to make n-type (excess electrons) or p-type (excess holes) in the material. LEDs emit light because of the materials selected so that the light is emitted in the ultra-violet, visible, or infrared ranges of the spectrum. The types of materials used are made from vapor deposition of materials on semiconductor wafers and cut into dice (a single one is a die). Typically, the die, or LED chips, are about 12 mil sq. The composition of the chips depends on the color, for example some red chips are AlInGaAs and some blue chips are InGaN. The variations are typically “three-five” variations, so-called because they vary based on the third and fifth period of the periodic table to provide the n- and p-type materials.
The conversion of an LED chip into an LED lamp is a costly process, involving very precise handling and placement of the tiny LED chip. The LED chips are most simply prepared as 3 mm LED lamps. The chip is robotically placed in a split cup with electrodes on each side. The entire structure is encased in a plastic lens that attempts to focus the beam more narrowly. High brightness chips may also be surface mounted with current-driving and voltage limiting circuits, and elaborate heat sink and heat removal schemes. Connection is by soldering or solderless ultrasonic wire bond methods. The result is a discrete point source of light. The LED lamp has a pair of leads, which can then be soldered to a printed circuit board. The cost of forming the lamp and then soldering the lamp to a printed circuit board is a relatively expensive process. Accordingly, there is a need to reduce the cost of forming a light emitting device based on the LED chip.
As an example application of LED lamps, it has recently been shown that ultraviolet LED lamps can be used to cure photo-polymerizable organic materials (see, for example, Loctite® 7700 Hand Held LED Light Source, Henkel-Loctite Corporation, Rocky Hill, Conn.
Photo-polymerizable organic materials are well known and are used for applications such as adhesives, binders and product manufacturing. Photo-polymerization occurs in monomer and polymer materials by the cross-linking of polymeric material. Typically, these materials are polymerized using radiation emitted from sources of light including intensity flood systems, high intensity wands, chambers, conveyors and unshielded light sources.
As an example use of photo-polymerizable organic materials, precision optical bonding and mounting of glass, plastics and fiber optics can be obtained with photo-polymerizable adhesives. These materials can be used for opto-mechanical assembly, fiber optic bonding and splicing, lens bonding and the attachment of ceramic, glass, quartz, metal and plastic components.
Among the drawbacks of the conventional systems that utilize photo-polymerizable organic materials is the requirement of a high intensity photo-radiation source. Typically, light sources, such as mercury vapor lamps, have been used to generate the radiation needed for photo-polymerization. However, these light sources are an inefficient radiation source because most of the energy put in to drive the lamp is wasted as heat. This heat must be removed from the system, increasing the overall bulk and cost. Also, the lamps have relatively short service life-times, typically around 1000 hours, and are very costly to replace. The light that is output from these light sources usually covers a much broader spectrum than the photo-radiation wavelengths that are needed for photo-polymerization. Much of the light output is wasted. Also, although the material can be formulated to be hardened at other wavelengths, the typical photo-polymerizable organic material is hardened at one of the peak output wavelengths of the mercury vapor lamp, to increase the polymerization efficiency. This peak output wavelength is in the UV region of the radiation spectrum. This UV radiation is harmful to humans, and additional shielding and protective precautions such as UV-filtering goggles are needed to protect the operators of such equipment.
FIG. 66 is a side view of an inorganic LED chip available. A conventional inorganic LED chip is available from many manufacturers, typically has a relatively narrow radiation emission spectrum, is relatively energy efficient, has a long service life and is solid-state and durable. The chip shown is an example of an AlGaAs/AlGaAs red chip, obtained from Tyntek Corporation, Taiwan. These chips have dimensions roughly 12 mil×12 mil×8 mil, making them very small point light sources. As shown in FIG. 67, in a conventional LED lamp, this chip is held in a metal cup so that one electrode of the chip (e.g., the anode) is in contact with the base of the cup. The metal cup is part of a cathode lead. The other electrode of the chip (e.g., the cathode) has a very thin wire solder or wiring bonding to it, with the other end of the wire solder or wiring bonding to an anode lead. The cup, chip, wire and portions of the anode and cathode leads are encased in a plastic lens with the anode and cathode leads protruding from the lens base. These leads are typically solder or wire bonded to a circuit board to selectively provide power to the chip and cause it to emit light. It is very difficult to manufacture these conventional lamps due to the very small size of the chip, and the need to solder or wire bond such a small wire to such a small chip electrode. Further, the plastic lens material is a poor heat conductor and the cup provides little heat sink capacity. As the chip heats up its efficiency is reduced, limiting the service conditions, power efficiency and light output potential of the lamp. The bulkiness of the plastic lens material and the need to solder or wire bond the lamp leads to an electrical power source limits emissive source packing density and the potential output intensity per surface area.
There is a need for a photo-radiation source that is energy efficient, generates less heat, is low cost and that has a narrow spectrum of radiation emission. There have been attempts to use inorganic light emitting diode lamps (LEDs) as photo-radiation sources. Usually, these LEDs are so-called high brightness UV radiation sources. A typical LED consists of a sub-millimeter sized chip of light emitting material that is electrically connected to an anode lead and a cathode lead. The chip is encased within a plastic lens material. However, the processing that takes the LED chips and turns it into an LED lamp is tedious and sophisticated, mostly due to the very small size of the LED chip. It is very difficult to solder or wire bond directly to the chips, and so it is common practice to use LED lamps that are then solder or wire bonded onto a circuit board. Conventionally, UV LED lamps have been solder or wire bonded onto a circuit board in a formation to create a source of photo-radiation for photo-polymerizable organic materials.
This solution is far from optimum, since the relatively high cost of the LED lamps keeps the overall cost of the photo-radiation source high. There is a need for a photo-radiation source that can use the LED chips directly, without the need for the lamp construction or a direct solder or wire bonded connection between the anode and cathode of the chip. Such as system would have an efficient chip packing density, enabling a high-intensity photo-radiation source having a narrow emission band.